The present invention relates to the fluoroorganic chemistry, particularly to a process for producing fluorinated aliphatic compounds by pyrolysis of perfluoro- and poly-fluorocarboxylic acids and their derivativesxe2x80x94halides and esters. The products of the pyrolysis, depending on the conditions under which it is carried out, are fluorinated olefins, perfluoroalkylvinyl ethers and polyfluoroalkanes. Fluorinated olefins and perfluoroalkylvinyl ethers are used as starting materials for obtaining polymer materials with improved operational characteristics, lubricating oils, elastomers, ion-exchange membranes for the electrolysis of aqueous solutions of alkali metal halides, etc. Polyfluoroalkanes, owing to their chemical inertness and thermal stability, find application as components of mix coolants, actuating fluids of thermocompressors, porophores in manufacturing foamed plastics and polyurethane foams, gas dielectrics, propellants, inert solvents, reagents for dry etching in manufacturing integrated circuits, and also in formulations of fire-extinguishing means.
At present a necessity is felt in the provision of an industrial process for producing fluorinated olefins, polyfluoroalkanes, perfluoroalkylvinyl ethers from different available organofluorine compounds under relatively mild reaction conditions with a high yield.
It is known that polyfluoroalkanes, particularly such as promising ozone-safe Freons 125, 227ea, are produced mainly by the hydrofluorination of perfluoroolefinsxe2x80x94tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), the production of which involves definite difficulties. TFE is explosion-hazardous, highly toxic, and self-polymerizes. TFE and HFP are produced by the high-temperature pyrolysis of chlorine- and fluorine-containing hydrocarbons, whose production in accordance with the decision of the Montreal Protocol concerning substances destroying the ozone layer is reduced because they deteriorate considerably the ecological situation. Therefore the possibility of obtaining polyfluoroalkanes (Freon 125, 227ea and the like) and olefins as such (TFE, HFP) from other available starting materials, namely, perfluoro- and polyfluorocarboxylic acids and their derivatives by a more simple process is a very urgent problem. The claimed process solves this problem.
It is known that in the pyrolysis of esters of perfluoro-alkoxycarboxylic acids of the formula
ICF2CF2OQCF(CF3)COOR,
where
Q is OCF2CF(CF3)[OCF(CF3)CF2]m, [OCF(CF3)CF2]n;
m is 0 to 7;
n is 1 to 4;
R is (C1-C6) alkyl
vinyl ethers are formed above a layer of carbonate, phosphate, sulfite, sulfate of an alkali or alkali-earth metal (U.S. Pat. No. 4,594,458, C07C 43/16, publ. 10.06.86). In this process, from an ester first a salt is obtained by treating NaHCO3 in a methanol solution, and then the salt is subjected to pyrolysis at 140-260xc2x0 C. In accordance with this process, perfluoro (8-iodo-4-methyl-3,6-dioxaoctene-1) is obtained with a yield of 71.8%.
A disadvantage of this process is the necessity of carrying out the intermediate step of obtaining an alkali metal salt and the necessity of drying this salt thoroughly, because the presence of moisture leads to the formation of by-products. Furthermore, toxic, fire- and explosion-hazardous methanol is used for carrying out the reaction.
In the pyrolysis of ethyl esters of pentafluoropropionic and heptafluorobutyric acid at 350-413xc2x0 C. and a pressure of about 8-80 gPa tetrafluoroethylene and hexafluoropropylene are formed, respectively, with a yield of about 30%, as well as an insignificant amount of pentafluorethane and heptafluoropropane (Int. J. Chem. Kinet., 1982, 14, No. 3, 291).
Known in the art is pyrolysis of polyfluoroalkoxyperfluoropropionic acid fluorides on sodium carbonate at 200-220xc2x0 C., which gives polyfluoroalkylperfluorovinyl ethers with a yield of 85-91% (Zhurnal Organicheskoi Khimii, 1978, 14, No. 3, 487). The pyrolysis proceeds in accordance with the following scheme:
RfCF(CF3)OCF(CF3)COFxe2x86x92RfCF(CF3)OCFxe2x95x90CF2,
where Rf is CF3CF2, CF3(CF2)3, CICF2CF2.
Na2CO3 is a reagent which is consumed in the process of the synthesis, forming NaF.
In the pyrolysis of polyfluorocarboxylic acid halides of the formula XCnF2nCOI (wherein X is H, F; n greater than 1; I is a haloid) at 200-500xc2x0 C., fluorolefins of the formula XCnF2nxe2x88x921 are formed (U.S. Pat. No. 3,020,321, 260-653.3, publ. 06.02.62). The reaction is carried out with oxides of Group IIA metals and silicon or with oxygen-containing salts of Group IA and Group IIA metals of the Periodic System.
The disadvantages of this process are a high reaction temperature (mainly 380xc2x0 C.) and direct participation of oxides and oxygen-containing salts in the process with the formation of inorganic fluorides, which makes the process unstable.
Decarbonylation of polyfluorocarboxylic acid fluorides is known in the art, which leads to obtaining polyfluoroalkanes in accordance with the following scheme:
H(CF2)nCOFxe2x86x92H(CF2)nF.
The reaction proceeds in the presence of a catalyst: anhydrous aluminum oxide (RU 659555, C07C 19/08, publ. 30.04.79) or under heating acyl fluoride with antimony pentafluoride (U.S. Pat. No. 3,555,100, C07C 19/08, publ. 12.01.71). As a result, 1-hydroperfluorohexane was obtained with a yield of 87-93%.
The prior-art processes cited above depend on starting materias, because monohydroperfluoroalkanes can be produced only from corresponding monohydroperfluorocarboxylic acid fluorides, which are not always available. They do not allow selective introduction of hydrogen into the carbon chain either.
High-temperature decarboxylation (620xc2x0 C. and 120 mm Hg) of 2-hydroperfluorobutyric acid gives 2-hydropentafluoropropylene with the yield of 92% (2-Hydropentafluoro propylene. Khim. Promyshl. Ser. Prikladnaya Khimiya, Moscow, NIITEK-hiM, 1979, pp. 1-2). The reaction proceeds in accordance with the following scheme:
CF3CHFCF2COOHxe2x86x92CF3CHxe2x95x90CF2.
Decarboxylation of xcex1-hydrohexafluoroisobutyric acid in dimethyl formamide gives 2,2-dihydrohexafluoropropane with the yield of 65% (Izv. AN SSSR, Ser. Khimiya, 1977, No. 5, 1112).
The reaction proceeds in accordance with the following scheme:
(CF3)2CHCOOHxe2x86x92CF3CH2CF3.
With the help of this process only dihydroperfluoroalkanes can be produced.
It is known that decarboxylation of fluorocarboxylic acids of the formula R(CFR1)n(CFR2)mOCF(CF2X)COOH, where R is SO2Z, POZ2, COZ; Z is OR3, F, Cl, Br, I; R3 is alkyl or aryl; R1 and R2 each are F, Cl, perfluoro- or chlorofluoroalkyl; X is Cl, Br, I; n is from 0 to 3; m is from 0 to 3, in the presence of an activator Na2CO3, ZnO or SiO2 in an organic solvent (mono-, di- or tetraglyme) at 50-150xc2x0 C. makes it possible to obtain corresponding vinyl ethers of the following formula: R(CFR1)n(CFR2)mOCFxe2x95x90CF2 (U.S. Pat. No. 4,358,412, C07F 9/113, publ. 09.11.82).
It is an object of the present invention to provide a universal industrial process for producing fluorinated aliphatic compounds with a high yield from available starting materials.
Another object of the present invention is to provide a process for producing fluorinated aliphatic compounds that are fluorinated olefins and perfluoroalkylvinyl ethers, by pyrolysis of perfluoro- and polyfluorocarboxylic acids, their halides and esters.
Still another object of the present invention is to provide a process for producing fluorinated aliphatic compounds that are polyfluoroalkanes, by pyrolysis of perfluoro- and polyfluorocarboxylic acids, their halides and esters in the presence of hydrogen fluoride.
Said objects are accomplished by the present invention, wherein a process for producing fluorinated aliphatic compounds is disclosed, consisting in carrying out pyrolysis of perfluoro- and polyfluorocarboxylic acids and their derivatives selected from the acid halides and esters, on a catalyst consisting of a carrier, most preferably selected from the series comprising active carbon, magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, nickel oxide, oxides of silicon promoted with alkali metal halides at a temperature of 100-450xc2x0 C., optionally in the presence of hydrogen fluoride.
Carrying out pyrolysis in the absence of hydrogen fluoride leads to the formation of fluorinated olefins and perfluoroalkylvinyl ethers. In the case of pyrolysis carried out in the presence of hydrogen fluoride polyfluoroalkanes are obtained as the target end products.
The process of pyrolysis is carried out continuously in a flow-through system, for which purpose a tubular reactor is used. The reactor is provided with electric heating, with a thermocouple sheath, with pipes for feeding the starting components and discharging the reaction products. The factor of filling the reactor with a catalyst is up to 0.8. The catalyst consists of a carrier preferably selected from the series comprising active carbon, silicon oxides, oxides of metals of Groups II, III, IV of the Periodic System, oxides of transition metals promoted with alkali metal halides. Compounds selected from the series comprising fluorides, chlorides, bromides, iodides of sodium, potassium, rubidium, cesium are used as said metal halides. It is most preferable to use magnesium oxide, calcium oxide, barium oxide, zinc oxide, aluminum oxide, nickel oxide as the above-indicated metal oxides. Examples of such catalysts can be KF/SiO2, NaF/SiO2, CsF/NiO, NaF/Al2O3, KF/CaO, CsF/SiO2, NaCl/Cact., KF/MgO, KI/Al2O3, etc. Active carbon and silicon dioxide, promoted with potassium fluoride, are the most preferable catalysts.
Optimal content of halides in the catalyst is from 20 to 50 wt. %. A reduction of the halide concentration to less than 20 wt. % leads to reducing the yield of the target product. An increase of the halide concentration above 50 wt. % does not influence substantially the process and is, therefore, inexpedient.
For preparing the catalyst, the carrier is mixed with an aqueous solution of an alkali metal halide, kept for up to 24 hours at room temperature, and then dried in a drying cabinet at 180-210xc2x0 C. to constant weight.
For creating a reaction zone, the prepared catalyst is charged into the reactor and heated in a stream of dry nitrogen, with the temperature increased gradually from 180 to 350xc2x0 C. for four hours. Then the starting reagent is fed into said reaction zone, said starting reagent being selected from the series comprising perfluorocarboxylic acids, polyfluorocarboxylic acids of a normal or isostructure, optionally containing a haloid other than fluorine, or their derivatives, such as acid halides or esters. Examples of said starting reagent can be perfluoropropionic acid, perfluorobutyric acid, perfluorovaleric acid, perfluoropelargonic acid, xcfx89-hydroperfluorobutyric acid, xcfx89-hydroperfluorovaleric acid, perfluoroisobutyric acid, perfluoropropionic acid chloride, perfluoropropionic acid fluoride, perfluoropropionic acid bromide, perfluoroisobutyric acid fluoride, perfluorobutyric acid chloride, perfluorovaleric acid fluoride, perfluorovaleric acid chloride, perfluoroenanthic acid chloride, perfluoropelargonic acid fluoride, xcfx89-hydroperfluorovaleric acid fluoride, xcfx89-hydroperfluoropropionic acid fluoride, perfluoropropoxyisopropionic acid fluoride, perfluoropropoxyisopropionic acid chloride, perfluoromethoxyisopropionic acid fluoride, 2-bromoperfluoroethoxyisopropionic acid fluoride, methyl ester of perfluoropropionic acid, ethyl ester of perfluoropropionic acid, methyl ester of perfluoroisobutyric acid, ethyl ester of perfluorovaleric acid, ethyl ester of perfluoroenanthic acid, methyl ester of perfluoropelargonic acid, methyl ester of xcfx89-hydroperfluorovaleric acid, ethyl ester of perfluoropropoxyisopropionic acid, methyl ester of perfluoropropoxyisopropionic acid, methyl ester of 2-bromoperfluoroethoxyisopropionic acid, methyl ester of perfluoropentoxyisopropionic acid.
The process of pyrolysis of the starting reagent in the presence of a catalyst is carried out in a sufficiently wide temperature range of 100 to 450xc2x0 C. At a temperature below 100xc2x0 C. the reaction slows down to such an extent that the conversion of the starting compounds does not exceed 5-10%. A temperature increase to 450xc2x0 C. causes destruction of the starting components and of the target products. For obtaining fluorinated olefins and perfluoroalkylvinyl ethers the temperature of 170-250xc2x0 C. is preferable. The synthesis of polyfluoroalkanes should be carried out preferably at a temperature of 250-350xc2x0 C.
The above-described catalytic pyrolysis of perfluorocarboxylic acids, polyfluorocarboxylic acids and their derivatives gives fluorinated olefins and perfluoroalkylvinyl ethers, such as, e.g., tetrafluoroethylene, hexafluoropropylene, perfluoro-1-butene, perfluoro-2-butene, perfluoro-2-pentene, perfluoro-2-hexene, perfluoro-2-octene, perfluoro-3-octene, perfluoro-4-octene, 1-hydroperfluoro-2-butene, trifluoroethylene, perfluoropropylvinyl ether, perfluoromethylvinyl ether, perfluoropentylvinyl ether, 2-bromoperfluoroethylvinyl ether.
In the case the above-described catalytic pyrolysis of perfluorocarboxylic acids, polyfluorocarboxylic acids and their derivatives is carried out with introducing hydrogen fluoride into the reaction zone, there takes place hydrofluorination of the fluorinated olefins and perfluoroalkylvinyl ethers which form as a result of the pyrolysis, and the obtained final products are polyfluoroalkanes containing at least one hydrogen atom, and ethers, for instance, 1-hydropentafluoroethane, 2-hydroheptafluoropropane, 2-hydroperfluorobutane, 1,3-dihydroperfluorobutane, 2-hydroperfluoroethylpropyl ether, 1-bromo-4-hydroperfluorodiethyl ether.
When producing polyfluoroalkanes, a 1.5-2.0-fold molar excess of hydrogen fluoride to the starting reagent is optimal. Feeding a smaller amount of hydrogen fluoride leads to an appreciable lowering of the yield of the target products, whereas an increase in the amount of the fed hydrogen fluoride is inexpedient.
Hydrogen fluoride can be introduced into the reaction zone simultaneously with the starting reagent or after performing the pyrolysis step. In this case hydrogen fluoride is fed to the reaction zone through the pipe into the middle part of the reactor.
The products of pyrolysis, i.e., fluorinated olefins and perfluoroalkylvinyl ethers, are condensed in a cooled collecting tank.
When producing polyfluoroalkanes, there remains unreacted hydrogen fluoride. To remove it, acid products of the pyrolysis are first washed with an alkali solution, then neutralized additionally on a column with a lime chemical absorber, and after that condensed at a temperature of xe2x88x9230 to xe2x88x9250xc2x0 C. The target products are isolated from the condensate by rectification and then identified by IR and NMR spectroscopy techniques.
In accordance with the present process there have been produced, in particular, 1-hydropentafluoroethane (Freon 125), 2-hydroheptafluoropropane (Freon 227ea) with the yield of 98%, and other polyfluoroalkanes, whose yield was not lower than 85%. In accordance with the present process there have been also produced various perfluoroolefins, polyfluoroolefins and perfluoroalkylvinyl ethers with a yield of up to 95%.